1. Field of the Invention
The present invention pertains to marine propulsion systems generally, and more specifically to marine propulsion systems utilizing an elongated propeller drive shaft having a housing commonly referred to as a drive tube surrounding the propeller shaft.
2. Description of the Related Art
Modern marine vehicles are most commonly powered by an internal combustion engine mounted within the boat or above the water line adjacent the boat. The mechanical power generated by the engine is transferred through a drive shaft to a water propulsion device such as a propeller. These marine vehicles provide a mode of transportation for traversing bodies of water that may be relatively large and open, such as the larger lakes, rivers and oceans, or relatively smaller, such as streams or creeks, swamps, glades, savannahs and the like.
For boating in open waterways such as lakes, rivers, or oceans, the propeller shaft is typically relatively short, and may extend from the motor and away from the boat hull only a few inches or feet. The spacing between propeller and hull in this type of boat is substantially smaller than the overall length of the boat. This short propeller shaft also dictates that the propeller is placed fairly deep into the water, to allow water to circulate past the boat hull and reach the propeller, and to avoid interference between propeller and boat hull during turns and the like. In open waters, where few if any obstacles exist, this arrangement has proven to be very effective and is represented by standard inboard and outboard marine propulsion systems.
Unfortunately, when traversing smaller or shallower bodies of water, such as swamps, creeks and streams, the rounded boat hulls and deep propeller arrangements used in open waterways are no longer effective or useful. The hull runs deeper than some sections of these smaller waterways, or obstacles present therein, and the propeller readily becomes tangled in vegetative matter, or, worse, may be destroyed by the obstacles. Particularly for those applications where the water is either shallow or filled with many obstacles, the prior art inboard and outboard motors are generally unsatisfactory.
To traverse the shallower bodies of water or those littered with obstacles, a generally flat bottom boat hull is preferred. In addition, the propeller drive shaft is extended beyond the boat by a much greater distance. When extended, the propeller can be driven shallowly in the water, free of interference with the boat. When an obstacle is encountered, the boat may pass over and be clear of the obstacle while still being propelled by the motor. Boats that use this type of drive system are sometimes referred to as mud boats, owing to their substantially improved propulsion in shallow waters, swamps, and other muddy waters. A number of US patents are illustrative of the prior art, the contents and teachings of each which are incorporated herein by reference, including U.S. Pat. No. 941,827 by Trouche, entitled “Motor more especially applicable for driving barges, wherries, flatboats, and the like”; U.S. Pat. No. 1,953,599 by Grimes, entitled “Boat propulsion device”; U.S. Pat. No. 2,096,223 by Chandler et al, entitled “Boat propelling mechanism”; U.S. Pat. No. 3,752,111 by Meynier, entitled “Pivoting motor boat drive unit”; U.S. Pat. No. 4,676,756 by Rodrigue et al, entitled “Boat and propulsion system including a transom platform”; U.S. Pat. No. 4,678,440 by Rodrigue et al, entitled “Boat and propulsion system”; and U.S. Pat. No. 8,911,272 by Gilk et al, entitled “Long Shaft Propeller Controller and Bearing Seal Protector”.
On propulsion systems having an extended drive shaft, there are a number of optional, but typically preferred additional features. For example, it is commonplace to use a housing or casing to surround the drive shaft, commonly referred to as a drive tube. Frequently, some type of shroud or structure such as a skag is also provided to prevent the propeller from directly striking any obstacles, and instead deflect the drive tube, drive shaft and propeller away from the obstacle. Additional features may be associated with the propeller and drive tube, such as various reinforcing elements, stiffeners or frameworks. The drive tube isolates the rotating propeller shaft from people and objects, thereby preventing the shaft from entangling or harming people or objects. The drive tube also protects the shaft from impact with hazards, and provides additional structural support to the drive shaft.
These long shaft drive systems commonly use a vertical axis pivot about which the long shaft and motor spin. This rotation allows the direction of propulsion to be varied. Commonly, the motor is balanced on one side of this vertical axis, and the long shaft, drive tube, shroud, and the like are on the opposite side. This balance also allows a horizontal axis pivotal coupling to be used at or near the balance point, and then the propeller can be raised or lowered relative to the water surface much like a children's see-saw in a playground.
One challenge associated with these long shaft drive systems is the variability in boat hull and transom design. A transom is the surface that forms the stern, or back, of the boat. The transom may be oriented in a vertical plane, but in most cases the transom will be raked aft, meaning the transom is typically either 12 or 16 degrees from vertical, with the bottom of the transom more forward or towards the bow than the top of the transom. In some less common designs, commonly referred to as a reverse transom, the bottom of the transom may instead be more rearward (aft) or farther from the bow than the top of the transom. Furthermore, transoms may be flat or curved transverse to the longitudinal axis of the boat, meaning that they may extend either more fore or more aft at the top center of the transom than at the top sides of the transom. In summary, transoms may be shaped very differently from one boat to the next. As a result, it is quite difficult to provide a single mounting system adaptable to the wide variety of transoms.
Not only is the transom variable from one boat to the next, hull designs are also often unique. Different hull designs and even different loading will create different patterns of water flow around the boat. Depending upon the boat hull, loading, and speed of travel, this can be quite important, since a wake is commonly created behind the boat with a crest of highly aerated water. A propeller is extremely inefficient in highly aerated water, and so it is very desirable to run the propeller in less disturbed water whenever possible.
Another challenge with long shaft drives is that many propeller mounts preferably have a vertical shaft that allows the engine and long shaft propeller to rotate freely. For typical use and operation, this is very desirable, since the operator will control the direction and elevation of the shaft to best move the boat through the water. Furthermore, and as noted above, when the long shaft or propeller shrouds encounter a submersed obstacle, it is also preferable to allow the shaft to move relatively freely to pass the obstacle without harm.
Unfortunately, the variability of the transom geometry and tilt from one boat to the next will lead prior art transom mounts to tilt at angles significantly offset from the desired vertical position. One serious drawback associated with the axis of rotation being offset from actual vertical is that, when turning a corner, the engine will tilt from side to side, essentially rotating or leaning around the propeller shaft axis. The normal roll about the horizontal axis which controls the height of the prop in the water, transverse to the propeller shaft axis, does not adversely affect engine lubrication and performance. However, if the engine rolls about the propeller shaft axis, the engine can run dry and be damaged.
In addition to the foregoing patents, Webster's New Universal Unabridged Dictionary, Second Edition copyright 1983, is incorporated herein by reference in entirety for the definitions of words and terms used herein.